Intracellular collagen fibrils were present in granular cells in atrioventricular valves of 5 patients with the syndrome of atrioventricular valve prolapse. These fibrils may form as a consequence of abnormal interactions between newly secreted collagen and acid mucopolysaccharides. BIBLIOGRAPHIC REFERENCES: Renteria, V. G., Ferrans, V. J., Jones, Michael, and Roberts, W. C.: Intracellular collagen fibrils in prolapsed ("floppy") human atrioventricular valves. Lab Invest 35: 439-443, 1976.